1. Field of the invention
This invention relates to a method for manufacturing a light-emitting material and, more particularly, to a method for manufacturing a light-emitting material capable of excitation at a low voltage.
2. Description of Related Art
Recently, nano-structure crystals in II-VI group semiconductors, such as micro-sized crystals represented by Si or Ge or porous silicon, are stirring up interest because they exhibit specific optical properties. The term nano-structure crystals means crystal grains having a crystal grain size of the order of several nm and are generally termed nano-crystals.
Comparison of the II-VI group semiconductors having nano-structure crystals to those having bulk structure crystals reveals that the nano-structure crystals exhibit satisfactory light absorption and light emission characteristics. This is possibly ascribable to the fact that the II-VI group semiconductor exhibits the quantization size effect and hence the band gap of the nano-structure crystals is larger than that of the bulk structure crystals. That is, with the II-VI group semiconductors having the nano-structure crystals, it is believed that the band gap is possibly widened by the quantization size effect.
Meanwhile, in a display for e.g., a television receiver, a variety of phosphors are used. The phosphors currently used in a television display are synthesized by firing the starting material at an elevated temperature. The light-emitting materials, thus synthesized, are of a particle size of the order of several micrometers (3 to 10 .mu.m). On the other hand, a thinner size of the television display is recently desired, such that there is an increasing interest in plasma display (PDP) which is a lightweight flat display, or an electroluminescence display (ELD).
With FED, to which particular attention is directed, the voltage of the electron beam needs to be lowered with decreasing thickness. However, if, with the display, thus reduced in thickness, the light-emitting materials with the particle size of the order of several .mu.m are used, sufficient light emission is not achieved because of the low voltage. That is, with the display, thus reduced in thickness, the conventional phosphors cannot be excited sufficiently.
This is possibly due to the fact that, since the conventional phosphors are of a large crystal structure, the illuminated electron beam cannot reach the light-emitting portion of the phosphors. Specifically, the conventional phosphors, with the particle size of the order of several .mu.m, cannot emit light sufficiently if used for a display with reduced thickness.
Among the phosphors emitting light at a low voltage, there is zinc oxide (ZnO:Zn) emitting green to blue light. Specifically, this ZnO:Zn is used in FED and can be excited at a low voltage of hundreds of volts to several thousands of volts. This ZnO:Zn is of a micro-sized crystal structure and electrically conductive such that it can emit light without charge-up even with a low voltage.
On the contrary, with the light-emitting materials having the nano crystal structure, the electron beam radiated at a low voltage can reach the light-emitting portion of the light-emitting materials. Thus, the light-emitting materials having the nano crystal structure can be used with advantage for the above-described display with the reduced thickness.
However, there is no other light-emitting material than the above-mentioned ZnO:Zn that is currently used as the light-emitting material capable of being excited at a low voltage. The manufacturing method for the II-VI group semiconductors having the nano crystal structure is not studied sufficiently such that a desired light-emitting material cannot be manufactured. Thus, there is a strong demand for a manufacturing method for a light-emitting material meritoriously used for a display with a reduced thickness.
On the other hand, the above-mentioned ZnO:Zn emits green to blue light, such that a II-VI group semiconductor emitting the other color light is desired as the light-emitting material. In addition, the II-VI group semiconductors having the nano crystal structure and which can be used with advantage not only for the FED but also with ELD and can emit light in various colors by the ultraviolet light are desired along with the manufacturing method for manufacturing these light-emitting materials.